In the third generation partnership project (3GPP) a proposal called D-TxAA is under discussion for UMTS as a way of increasing the peak bit rate. This is derived from an existing closed loop transmit diversity scheme (TxAA mode 1) where the mobile terminal signals to the network complex weights which should be applied to the signals from each of two transmitting antennas. In D-TxAA two different data streams are transmitted using orthogonal weight vectors, wherein a first weight vector is based on those transmitted from the mobile terminal, and a second vector is derived deterministically from the first vector.
For the operation of D-TxAA, the following may be assumed:
Orthogonal pilot channels are transmitted from an antenna of each Node B (which is a logical node responsible for radio transmission and reception in one or more cells to and from an user equipment (UE).
No dedicated (i.e. beam formed) pilots are available (assuming that the fractional dedicated physical channel (F-DPCH) is used, which does not carry pilot bits).
Feedback information (FBI) for the first beam is derived by the user equipment (UE) and transmitted to Node B, indicating the desired beamforming vector.
The first beam is transmitted using a restricted codebook of weight vectors (for example the codebook currently used for TxAA mode 1).
The identity of the antenna weight vector for a first beam is signaled to the UE on the High-Speed Shared Control Channel (HS-SCCH).
The second beam is transmitted using a deterministic phase vector, which is orthonormal to the vector for the first beam.
Channel quality information (CQI) is signaled by the UE to the Node B, enabling the Node B to derive a different rate for each of the two beams.
The CQI indicates the rate (or packet size) which can be transmitted successfully (or with a given probability of success) using a reference power level and code resource (the reference values being known by both the network and the mobile terminal).
The transmissions on the two beams are comprised of separate codewords with potentially different rates.
As the simultaneously transmitted beams in D-TxAA are typically received with different SINR (signal-to-noise ratio where the noise includes both thermal noise and interference) at the UE, each beam can support a correspondingly different rate. This implies that multiple CQI information is required to be signaled to the Node B by each UE. In UMTS Release 5, a single CQI value is comprised of 5 information bits, coded into 20 physical channel bits. For a multiple-beam system, this number of bits would be multiplied by the number of beams if a separate CQI value is indicated for every beam. This can result in a high signaling load.